The ability to detect constituents such as particles or trace chemicals in a gas offer numerous applications. For example, it is known that a hidden cache of explosive material sheds trace amounts of materials into the air. Similarly, the presence at the site of a recent fire of certain organic compounds in the air is evidence of an arsonist's use of an accelerant. Detection of airborne bacteria, viruses, molds and other living matter can assist epidemiologists in monitoring and controlling the spread of disease.
The detection of airborne constituents by known techniques generally involves two steps. First, the constituents are collected, and second, they are analyzed. Analysis can be performed, for example, by passing the sample through a mass spectrometer, by gas chromatography, or even visually. The collection step generally requires that the constituents be made available in sufficient concentration for the analysis step to be performed. The sufficiency of concentration depends on the sensitivity of the analyzer and on the nature of the constituent to be detected.
Where the concentration of the selected constituent in ambient air is insufficient to permit the analyzer to function, the selected constituent must first be concentrated before they can be analyzed. Conventional methods of concentrating selected constituents include passing air through a filter and, after sufficient time has elapsed to build up the required concentration of constituents on the filter, collecting the residue from the filter for analysis.
Because in practice the concentration of particles in air is extremely low, it becomes necessary to pass large volumes of air through the filter in order to collect sufficient residue on a filter for analysis. The time required to pass large volumes of air through the filter and the additional time required to remove the residue from the filter and to prepare it for analysis make this method impractical where rapid response is necessary.
The air sampling process can be accelerated to some extent by increasing the volume rate of flow through the filter. However, the mechanical resistance of a filter to fluid flow increases as the fluid velocity increases. As a result, the energy required to force the fluid through the filter increases. Moreover, if the fluid velocity is too high, the fluid force on the filter may cause the filter to rupture.
This conventional method of concentration is unable to concentrate trace gases from ambient air because such gases can generally pass through the filter. Moreover, where the particles are microorganisms, the large volume of air can desiccate any microorganisms already trapped by the filter, thereby destroying the viability of the sample.
Consequently, there is a need in the art for a device which can rapidly and efficiently collect and concentrate airborne constituents of a gas stream for subsequent analysis.